1. Field of the Invention
It has been determined that continued problems in processing high temperature resistant aromatic heterocyclic polymer systems may be lessened by the incorporation of hexafluoroisopropylidene (6F) units in the polymer backbone structure. These 6F units tend to lower the glass transition temperature (Tg) of polymers prepared therewith and thus improve their melt characteristics. In addition, 6F units within a polymer chain prohibit extensive conjugation of aromatic moieties, thereby providing good electric insulation characteristics for the end product resins. Such lack of conjugation also obliterates UV-visible chromophores, thus allowing end product polyimides to appear colorless and to resist photochemical degradation.
Key monomers for the synthesis of thermally stable high temperature resins containing 6F groups have been prepared from commercially available 2,2-bis(4-hydroxyphenyl)hexafluoropropane, commonly known as Bisphenol AF. However, because of the resistance of the hydroxy functions on Bisphenol AF to replacement by direct chemical modification, the availability of such monomers is severely limited to monomers having substituents which are less reactive than halogen substituents.
Diphenyl derivatives of hexafluoropropane have proven to be useful intermediates in the synthesis of thermally stable resins for use in high temperature structural composites. Halogenated 2,2-diphenyl derivatives of hexafluoropropane can be predicted to be particularly useful in that the halogen substituents will render the derivative more reactive and thus easier to utilize in preparing high temperature resins. 2,2-Bis(4-bromophenyl)hexafluoropropane is a novel species of this class of compounds that has a great potential for use as an intermediate in the synthesis of high temperature resins for colorless coating applications. Therefore, this invention is directed to the provision of novel bis(halophenyl)hexafluoropropane compounds and an improved process for making bis(halophenyl)hexafluoropropane compounds.
2. Description of the Prior Art
Bisphenols in general and 2,2-bis(4-hydroxyphenyl)hexafluoropropane (Bisphenol AF) in particular have been used by others as starting materials for the synthesis of intermediates used in the preparation of high temperature resins. These materials are commercially available. However, the transformation of the hydroxy substituents found on the phenylene rings of these bisphenols into other substituents is difficult because phenols in general are inert towards direct displacement reactions. In the case of Bisphenol AF, which contains the inductively electron withdrawing hexafluoroisopropylidene (6F) group in addition to the relatively inert hydroxyl groups, the synthesis of intermediates containing halogen substituents on the phenylene ring is particularly difficult and inefficient.
Attempts to prepare 2,2-bis(4-bromophenyl)hexafluoropropane using phosphorus pentabromide as a brominating agent yield an intractable product mixture, as reported by Dr. K. L. Paciorek, et al. of Ultrasystems, Incorporated, of Irvine, Calif.; see NASA Report No. CR-159403 entitled "Synthesis of Perfluoroalkylene Diamines" that was submitted under NASA Lewis Research Center Contract NAS3-20400 August 1978. It can be concluded from this work that there are no suitable prior art methods for preparing halogen substituted diphenyl derivatives of hexafluoropropane.
Therefore, there is still a need for an efficient process for preparing 2,2-diphenyl derivatives of hexafluoropropane having halogen substituents thereon and there is particularly a need for para-substituted 2, 2-diphenyl derivatives of hexafluoropropane.